An example of a spindle head for a machine tool includes a fork which rotates around an A-axis while holding a spindle unit, which rotates a tool, at both sides of the spindle unit and a driving unit (hereinafter referred to as a C-axis driving unit) which rotates the fork around a C-axis (Patent Document 1).    Patent Document 1: U.S. Pat. No. 5,584,621
The above-described C-axis driving unit includes a rotary shaft including an inner sleeve secured to the fork, a bearing which supports the rotary shaft (inner sleeve) such that the rotary shaft is rotatable, and a drive motor for rotating the rotary shaft, all of which are accommodated in a housing. The drive motor includes a rotor (motor rotor) secured to the rotary shaft and a stator (motor stator) secured to the housing such that the stator surrounds the rotor. Thus, the C-axis driving unit included in the spindle head according to Patent Document 1 includes a direct drive motor (so-called DD motor) for driving the rotary shaft. In addition, the rotor and the stator included in the drive motor are accommodated, together with a portion of the rotary shaft, in the housing such that three sides thereof, that is, the peripheral, top, and bottom sides thereof are surrounded. The C-axis driving unit serves as an index device which rotates the rotary shaft (fork) to a desired angular position using the drive motor.
Each of the above-described C-axis driving unit included in the spindle head, the above-mentioned rotary table device, etc., is assembled to a machine tool as an index device to form a component of the machine tool, and is generally used while being supported by the machine tool such that a component thereof which corresponds to a housing (frame) is attached to the machine tool at a suitable position. In addition, although not described in Patent Document 1, the above-described index device is generally provided with a clamp mechanism for maintaining an angular position of a rotation object (e.g., the fork in the spindle head) after indexing the angular position.
When a machine tool is used for a predetermined period of time, it becomes necessary to perform maintenance for the bearing, the clamp mechanism, etc., included in the above-described index device. However, it is extremely difficult to perform the maintenance without removing the index device from the machine tool. Therefore, according to the related art, the entire body of the index device is removed from the machine tool to perform the maintenance. For example, in the above-described spindle head, the entire body of the C-axis driving unit is removed from the machine tool.
However, the spindle head according to Patent Document 1 has a problem that the process of removing the entire body of the C-axis driving unit from the machine tool is cumbersome. More specifically, the C-axis driving unit included in the above-described spindle head according to Patent Document 1 includes the DD motor as drive means thereof, and a plurality of cables (e.g., a current supplying line, a ground line, and a line for a detector) are connected to the motor stator included in the DD motor. The cables are arranged such that the cables extend to the outside of the housing, and are connected to the corresponding wiring lines provided in the machine tool with connectors. Therefore, the cables must be disconnected from the wiring lines provided in the machine tool in the process of removing the C-axis driving unit from the machine tool. In addition, after the maintenance, the cables must be reconnected to the wiring lines in the process of reattaching the C-axis driving unit to the machine tool. The process of connecting and disconnecting each wiring line must be repeated the same number of times as the number of wiring lines, and therefore this process is extremely cumbersome and takes a long time. The process of connecting and disconnecting the wiring lines becomes more cumbersome as the number of drive motors increases.